Signaling system



March 15, 1932.

Filed May 26, 1928 $53 NQCRQQR Q.-K% RP-$Q W Y x m D m T R IV lllll lllOll llllllllll llll W L m x w M m II II 11v 1 6 0 DA I 1| lllllll m w M MCN ll illli l Patented Mar. 15, 1932 ATES PATENT @FFEQE ROBERT L. DAVIS, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOE TO \FIESTINGEGUSE SIGNALING SYSTEM- Application filed May 26,

My invention relates to signaling systems, and more particularly to radio signaling systems of the type wherein high-power thermionic tubes are employed for the generation and control of oscillatory currents at radiofrequencies.

At the present time, substantially all radioprogram transmission is accomplished by causing the amplitude of a radiated carrierwave to vary in accordance-with the speech or music being transmitted. The preferred means for efiecting the amplitude variation, or modulation, of the carrier wave is generally credited to Heising, and comprises a thermionic oscillation generator and'a thermionic modulator, the anodes of which are supplied with potential from acommon source. An audio-frequency choke coil is included in the common lead to the anodes of the oscillator and modulator devices, in order that the current taken thereby shall remain constant. When signal frequencies are impressed on the input circuit of the modulator device,

therefore, the potential applied to the anode of the oscillator varies in accordance therewith, thus causing th amplitude of the generated and radiatedoscillations to vary proportionally to'the said signal frequencies.

It is substantially imposs ble to secure-complete modulation with the Heising constantcurrent system, however, since the entire cessation of oscillations in the oscillator deice pro-supposes zero potential on the plate thereof and also zero potential on the plate of the modulator device during the intervals the latter is drawing maximum plate current. It has, accordingly, been proposed to operate the oscillator tubes a lowerplatepotential than the modulator tubes in such mannor that. when the oscillator plate potential falls to zero, for 100% modulation, there is still sufficient residual plate potential on the modulator tubes to permit them to draw current.

Several systems have been devised for accomplishing modulation in the manner just referred to but all of them with which I am familiar have the disadvantage of requiring separate and distinct high-potential supplies for the oscillator and modulator tubes.

1928. Serial No. 289,696.

It is, accordingly, an object of my invention to provide a signaling system comprislng means for accomplishing the complete modulation of the outputthereof in accordance with signals at audio, or other, frequencies.

Another object of my invention is 'to provide asignaling system comprising an oscillation generator and means for securing 100% modulation of the output from said gener ator.

Another object of my invention is to provide a signaling system comprising an oscillation generator of the thermionic type, a modulating device therefor, and means whereby the said modulating device may so controlsaid oscillation generator as to secure 100% modulation of the output thereof.

Another object of my invention is to provide a signaling system wherein 100% modulation shall be accomplished without wasting large amounts of energy.

Still another and more specific object of my invention is to provide an inter-stage coupling network particularly adapted for use in connection with signaling systems wherein audio-frequency currents, at large amplitudes, are handled.

In carrying my invention into effect, 1 provide a thermionic oscillator and a thermionic modulator with a single source of anode potential, the common connection to the anodes of the several devices includin the customary audio-frequency choke coil. Instead of maintaining the cathodes of the oscillator and modulator tubes at the same potential, however, and directly connecting the negative terminal of the anode-potential source thereto, I provide means for keeping the cathodes of the oscillator tubes positive with respect to the said negative terminal, and the cathodes of the modulator tubes negative with respect thereto.

I am, therefore, enabled to secure all the benefits of separate sources of oscillator and modulator plate-potential without the necessity of employing a plurality of expensive high-potential generators therefor, an inexpensive motor-generator set being amply sufficient to supply the necessary difference of potential bet-ween the cathodes of the oscillator tubes and the cathodes of the modulator tubes.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method of operation, together with further objects and advantages thereof, will best be understood by reference to the followingdescription of a specific embodiment, taken with the accompanying drawings, in which:

Figure 1 is a diagrammatic View of a complete radio signaling system comprising a preferred embodiment of my invention, and

Fig. 2 is a diagram illustrating the relation between modulator grid-potentials and oscillator plate-potentials which obtains in a signaling system comprising my invention.

The signaling system illustrated in Fig. 1 may, for purposes of convenience in description, be divided into a modulator-oscillator portion which handles oscillatory currents at high frequencies and high potentials and a speech-amplifier portion which handles only currents at audio or other low frequencies.

The amplifier portion comprises a microphone 1 or an analogous device, which is connected in series with the primary winding 2 of an input transformer, the secondary winding 3 of which is connected between the grid and filament of a low-power amplifier tube 4. Plate. potential for the amplifier tube is supplied from a source 5 over a circuit including an audio-frequency choke coil 6.

A plurality of amplifier tubes 10 and 11, of slightly higher power than the first amplifier tube 4', are connected in push-pull or balanced relation and are supplied with filament potential from a source 12, which source also supplies filament potential for the first amplifier tube. The input circuit for the tubes 10 and 11 comprises an auto-transformer 13, an intermediate point on which is connected through a source 14 of grid-biasing potential to the cathodes of the said tubes. The wind-- ings of the auto-transformer are preferably inter-leaved to reduce the leakage reactance thereof.

One terminal of the auto-transformer is connected, through a large condenser, to the plate of the first amplifier tube, whereby the potentials at audio-frequency which are developed across the choke coil 6 are impressed across that winding of the auto-transformer included between the grid of the amplifier tube 10 and the filament thereof.

The output circuit for the amplifier tubes 10 and 11 comprises the primary winding 15 of an'output transformer, an intermediate point on the primary winding being connected to the potential source 5 in the customary manner.

I have thus provided a very convenient means whereby a stage of ordinary singletube amplification may be coupled to a pushpull power amplifier without the necessity of using an expensive double-winding transformer. In addition, inasmuch as the leakage reactance of the auto-transformer may be made extremely low, an amplifier constructed according to my invention has much better characteristics than when the usual three-winding push-pull transformer is used as a coupling device. 3

The modulator-oscillator portion of the signaling system preferably comprises a high-power water-cooled thermionic modulator tube 20 which is supplied with filament potential from a small generator 21. The secondary winding 22 of the push-pull output transformer is included between the grid of the modulator tube and an intermediate point on a balancing resistor 23 connected in shunt to the filament generator 21. A source 24 of biasing potential is preferably included in the input circuit of the modulator tube in order that the grid thereof may be maintained at the best operating point.

An oscillator tube 30 having a grid 31, a

plate 32, and a filament 33, is supplied with filament potential from a small generator 34. A resistor 35 is connected in shunt to the terminals of the generator 34, and an intermediate point thereon is connected to the intermediate point on the resistor 23 shunting the modulator-filament-potential generator over a circuit including a plurality of small generators 36 and 37, the armatures of which are mechanically interconnected and arranged to be driven by a constant-speed motor 38 on a main motor-generator set.

The positive terminal of a common source 40 of high potential is connected to the plates of the modulator and oscillator tubes through an audio-frequency choke coil 41, while the negative terminal of the source is connected to an intermediate point on a conductor 42 which connects the generators 36 and 37.

The intermediate point is grounded to main tain it at zero potential.

The oscillator tube 30 is provided with an oscillatory output circuit comprising an inii ductor 43 having a tuning condenser 44 connected in shunt thereto. Separated points on the inductor 43 are connected to an inductor 45 comprised in a radiating structure or to later amplifier stages (not shown). The

grid 31 and the plate 32 of the oscillator tube are connected to opposite terminals of the output inductor 43 through a plurality of blocking condensers 46 and 47, and an intermediate point on the output inductor is connected,by a conductor 48, to the resistor shunting the oscillatortube filament-potential generator.

In the operation of the system illustrated, the balancing set, comprising the generators 36 and 37, maintains the filaments of the modulator and oscillator tubes below and above ground potential, respectively. The space current in the oscillator tube flows against the potential developed by the generator 06, causing the latter to act as a motor to supply power to the generator 37, in addition to the power supplied thereto from the driving motor. The generators 3637, therefore, serve as energy-converting devices, functioning to maintain a greater potential difference between the plate and filament oi the modulator tube than that between the plate and filament of the oscillator tube.

By proper adjustment of the field control (not shown) of the balancing-set machines and the oscillator and modulator plate currents, the driving motor 38 could be dispensed with. However, in order to provide for greater latitude in adjustment and improved stability, it is desirable to make use of the driving motor, as shown. In some instances, it is possible that the driving motor might be required to absorb energy and to return it to the line.

In a signaling system, as actually con structed, the modulator and oscillator tubes would, in all probability, be replaced by a plurality of similar tubes connected in parallel, in order that the requisite power output might be obtained without distortion.

In order that the theory underlying my invention may be thoroughly understood, reference should be made to Fig. 2.

In the diagram, distances measured along the X axis, to the left of the intersection thereof with the Y axis, represent potentials impressed upon the modulator grid from the preceding speech amplifier. Distances measured along the Y axis represent plate potentials with respect to the X axis which is representative of Zero or ground potential.

As the modulator-grid potential swings more or less negative, as indicated by the curve A, the plate potential thereon varies along the line BC above and below the normal plate potential, as shown by the curve D. By reason of the fact that the oscillator-cathode potential is positive with respect to the negative terminal of the high-potential source, which terminal is maintained at ground potential, the common plate potential may be reduced to the said cathode potential, as illustrated, to cause the cessation of oscillations, while still being sufiiciently positive with respect to the modulator cathode to permit space current to flow in the modulator tube. It is, accordingly, apparent that, by my invention, I have provided a system wherein complete modulation of the oscillator output may be obtained wi hout the necessity of employing a plurality of separate high-potential sources for the plates of the modulator and oscillator tubes, as has heretofore been proposed.

In certain instances, when the weight, the space required, or the cost of the balancing generators is a disadvantage, they may be dispensed with and analogous results obtained by inserting a resistor shunted by a large condenser in series between the plate of the modulator tube and the terminal of the choke coil which is common to both modulator and oscillator plates. Energy, however,

is lost in the resistor because heat is not returned to the system, as it is when the balancin set is employed.

It is also feasible to dispense with the generator 37 and the driving motor 38 and, instead, to connect the generator 36 through an appropriate gearing or belting to the main high-potential generator 40, or to a filamentsupply generator in order to return power to the system. In addition, it is within the province of my invention to apply my improved modulation system to a power amplifier stage, instead of to a self-excited oscillator, the modulation of the amplifier being accomplished in substantially the same manner as hereinbefore described.

My improved signaling system is principally advantageous in that, by its use, 100% modulation of the output of an oscillation generator or a power amplifier may be obtained while, at the same time, a lesser number of modulator tubes is required than in systems known to the prior art.

The balancing set, or the resistor in the alternative form mentioned, is far less expensive than separate plate-potential sources for the modulator and oscillator tubes, or for the modulator and power-amplifier tubes if modulation is accomplished in an amplifier stage succeeding a master-oscillator.

My improved coupling circuit for a single tube amplifier and a push-pull amplifier is described and claimed in my application, Ser. No. 584,497, filed January 2, 1932, which is a division of this application.

Although I have illustrated and described only a single embodiment of my invention, many modifications, in addition to those hereinbefore referred to, as well as additional advantages thereof, will be apparent to persons skilled in the art. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1.. In combination, a plurality of thermi onic devices having space-current paths connected in parallel across a common high-potential source and an audio-frequency choke coil in series, an energy-translating device interposed between said source and one of said devices, and means whereby said energytranslating device operates to maintain the cathode of said last-mentioned device at a difierent potential with respect to the negative terminal of said potential source than the potential of the cathode of another of said devices with respect to said negative terminal.

2. In combination, a plurality of thermionic devices having space-current paths connected in parallel across a common high potential source and an audio-frequency choke C011 in series, an energy translating device interposed between said source and one of said devices, and means whereby said energy-translating device operates to maintain the cathodes of said devices at difierent potentials with respect to ground potential.

3. In combination, a plurality of thermionic devices'having space-current paths connected in parallel across a common high-potential source and an audio-frequency choke coil in series, an energy-translating device interposed between the cathode of one of said devices and the negative terminal of said source, and means whereby said energytranslating device serves to maintain the anode of one of said devices at a. higher potential with respect to the cathode thereof than the potential of the anode of the device with which said energy-translating device is associated with respect to its cathode.

4:. In combination, a plurality of thermionic devices having space-current paths con nected in parallel across a common high-potential source, a motor interposed between the cathode of one of said devices and the negative terminal of said source, and means actuated by said motor for increasing the potential applied between the anode and oathode of another of said thermionic devices.

5. In combination, a plurality of thermionic devices having space-current paths connected in parallel across a common high-potential source, a motor interposed between the cathode of one of said devices and the negative terminal of said source, a generator interposed'between the cathode of another of said devices and said negative terminal, and a mechanical connection between said motor and said generator.

6. In combination, a plurality of thermionic devices having space-current paths connected in parallel across a common high-p0 tential source and an audiodfrequency choke coilin series, a motor interposed between the cathode of one of said devices and the negative terminal of said source, a generator interposed between the cathode of another of said devices and said negative terminal, a mechanical connection between said motor and said generator.

In testimony whereof, I have hereunto subscribed my name this 14th day of May, 1928.

RGBERT L. DAVIS. 

